The XHTML form itself is the interface for the user to enter data, but all of the actual data processing is performed on the server using applications that usually reside in the Common Gateway Interface (CGI).

The Common Gateway Interface is the communications bridge between the Internet and the server. Using programs called “scripts,” CGI can collect data sent by a user via the Hypertext Transfer Protocol (HTTP) and transfer it to a variety of data processing programs, including spreadsheets, databases, or other software running on the server. The data processing software then can work with the data and send a response back to CGI, and then on to the user.